1. The Field of the Invention
The present invention relates to a method for determining the suitability of an optical material for the production of optical elements, to a device for carrying out the method, and to the optical elements made from the optical materials determined to be suitable using the method.
2. Related Art
It is known that materials used to produce optical elements absorb irradiating light to a greater or lesser extent, so that the intensity of the light and/or radiation is typically less after it passes through an optical element than before it passes through. It is also known that the extent of this absorption depends on the wavelength of the light. For optical systems, i.e., for optically transparent systems, the goal, however, is to keep the absorption as low as possible, i.e., they should have a high light permeability or transmission, at least for the particular working wavelength. The absorption is composed of material-specific (intrinsic) portions and portions due to “non-intrinsic” portions, such as inclusions, contamination and/or crystal imperfections. While the intrinsic absorption is independent of the particular quality of the material, the additional radiation absorption of the non-intrinsic absorption results in a degradation of the optical material.
As a result of the intrinsic and non-intrinsic absorption, energy is deposited in the optical material, which results in a temperature rise. The disadvantage of the material heating in this manner is that the optical properties, e.g., the refractive index, change, which results in a change in the reproduction ratios in an optical component used for beam shaping, for example, since the refractive index depends not only on the wavelength of the light, but also on the temperature of the optical material. In addition, a temperature rise in an optical component also results in a change in the lens geometry. These phenomena produce a change in the lens focal point and blurriness in images projected with the heated lens. In photolithography in particular, which is used to produce computer chips and electronic circuits, this results in quality degradations and/or an increase in waste, and is therefore not desired.
With many materials, a portion of the absorbed radiation is not only converted to heat, but is also given off again in the form of fluorescence. The formation of fluorescence on optical materials, in particular on optical crystals, is known per se. For example, W. Triebel et al. describe, in Proceedings SPIE Vol. 4103, pages 1-11, 2000 Triebel, Bark-Zollmann, Mühlig et al. in “Evaluation of Fused Silica for DUV Laser Applications by Short Time Diagnostics”, the formation and measurement of laser-induced fluorescence (LIF) in quartz, particularly in OH-rich quartz and/or a glass matrix. Furthermore, M. Mizuguchi et al. describe, in J. Vac. Sci. Technol. A., Vol. 16, pages 2052-3057 (1998), the formation of optical absorption bands in a calcium fluoride crystal. In addition, M. Mizuguchi et al. describe, in J. Opt. Soc. Am. B, Vol. 16, pages 1153-1159, July 1999, a time-resolved photoluminescence for diagnosing the laser damage done to a calcium fluoride crystal. This article describes the formation of color centers that form photoluminescence via excitation with an ArF excimer laser at 193 nm. To enable measurements of this type, however, crystals with a relatively high amount of impurities are used in this case, and this does not fulfill the high requirements for photolithography. In addition, the fluorescence measurement is carried out in the sample to be investigated after a waiting period of 50 nsec after the laser pulse ends. It has been shown that the fluorescence values obtained in this manner cannot be used for quality control purposes or to determine the extent of the impurity, and therefore cannot be used to form color centers in the high-quality crystals.
It is therefore believed that the determination of radiation-induced fluorescence, in particular laser-induced fluorescence, cannot be used for quality control of high-quality, optical materials, as with high-purity calcium fluoride for photolithography, for example. (Refer also to the presentation by Dr. Mann, Laserlabor Göttingen, SPIE Conference in Seattle, USA, July 2002). It was determined that a correlation cannot be made between laser-induced fluorescence and a claim regarding impurities and the optical quality of a material.